Wireless power system with reconfigurable rectifier circuitry

ABSTRACT

A wireless power system has a wireless power transmitting device and a wireless power receiving device. The wireless power transmitting device may be a wireless charging mat or other device with coils for transmitting wireless power signals. The wireless power receiving device may be a cellular telephone or other device with coils for receiving the transmitted wireless power signals. The wireless power receiving device has adjustable rectifier circuitry coupled to a pair of coils. The pair of coils is coupled in series at a node. A transistor is coupled between ground and the node and is controlled by control circuitry. The state of the transistor can be changed to place the adjustable rectifier circuitry in either a first mode of operation in which the adjustable rectifier circuitry forms a full-bridge rectifier or a second mode of operation in which the adjustable rectifier circuitry forms a pair of parallel half-bridge rectifiers.

This application is a continuation of patent application Ser. No.16/505,370, filed Jul. 8, 2019, which claims benefit of provisionalpatent application No. 62/828,933, filed Apr. 3, 2019, both of which arehereby incorporated by reference herein in their entireties.

FIELD

This relates generally to power systems, and, more particularly, towireless power systems for charging electronic devices.

BACKGROUND

In a wireless charging system, a wireless power transmitting device suchas a charging mat wirelessly transmits power to a wireless powerreceiving device such as a portable electronic device. The portableelectronic device has a coil and rectifier circuitry. The coil of theportable electronic device receives alternating-current wireless powersignals from the wireless charging mat. The rectifier circuitry convertsthe received signals into direct-current power.

SUMMARY

A wireless power system has a wireless power transmitting device and awireless power receiving device. The wireless power transmitting deviceis a wireless charging mat or other device with coils for transmittingwireless power signals. The wireless power receiving device is acellular telephone or other device with coils for receiving thetransmitted wireless power signals.

To enhance wireless power transmission efficiency in a variety ofoperating scenarios, the wireless power receiving device may configurethe coils in the wireless power receiving device to receive magneticflux that is transmitted from the wireless power transmitting device ina first orientation (e.g., horizontal flux) or a second orientation(e.g., vertical flux).

The wireless power receiving device has adjustable rectifier circuitrycoupled to a pair of coils. When receiving wireless power, a first ofthe coils can produce alternating-current signals that are in phase orthat are out of phase (e.g., 180° out of phase) with respect to a secondof the coils depending on the orientation of the transmitted magneticflux. The adjustable rectifier circuitry is dynamically reconfigured toaccommodate these different scenarios.

The first and second coils are coupled in series at a node. A transistoris coupled between ground and the node. The transistor is controlled bycontrol circuitry. The state of the transistor can be changed to placethe adjustable rectifier circuitry in either a first mode of operationin which the adjustable rectifier circuitry forms a full-bridgerectifier or a second mode of operation in which the adjustablerectifier circuitry forms a pair of parallel half-bridge rectifiers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an illustrative wireless chargingsystem that includes a wireless power transmitting device and a wirelesspower receiving device in accordance with an embodiment.

FIG. 2 is a circuit diagram of wireless power transmitting and receivingcircuitry in accordance with an embodiment.

FIG. 3 is a side view of an illustrative wireless power transmittingdevice such as a wireless charging pad and a corresponding wirelesspower receiving device such as a cellular telephone with multiplewireless power receiving coils in accordance with an embodiment.

FIG. 4 is a circuit diagram of illustrative adjustable rectifiercircuitry in accordance with an embodiment.

DETAILED DESCRIPTION

A wireless power system includes a wireless power transmitting devicesuch as a wireless charging mat. The wireless power transmitting devicewirelessly transmits power to a wireless power receiving device such asa wristwatch, cellular telephone, tablet computer, laptop computer, orother electronic equipment. The wireless power receiving device usespower from the wireless power transmitting device for powering thedevice and for charging an internal battery.

The wireless power transmitting device communicates with the wirelesspower receiving device and obtains information on the characteristics ofthe wireless power receiving device. In some embodiments, the wirelesspower transmitting device has multiple power transmitting coils. In suchembodiments, the wireless power transmitting device uses informationfrom the wireless power receiving device and/or measurements made in thewireless power transmitting device to determine which coil or coils inthe transmitting device are magnetically coupled to wireless powerreceiving devices. Coil selection is then performed in the wirelesspower transmitting device.

Wireless power is transmitted from the wireless power transmittingdevice to the wireless power receiving device using selected coil(s) tocharge a battery in the wireless power receiving device and/or to powerother load circuitry. The wireless power receiving device hasreconfigurable coils. For example, the wireless power receiving devicemay have a pair of coils coupled to adjustable rectifier circuitry. Therectifier circuitry can be operated in different modes to enhancewireless power reception by the coils.

An illustrative wireless power system (wireless charging system) isshown in FIG. 1. As shown in FIG. 1, wireless power system 8 includes awireless power transmitting device such as wireless power transmittingdevice 12 and includes a wireless power receiving device such aswireless power receiving device 24. Wireless power transmitting device12 includes control circuitry 16. Wireless power receiving device 24includes control circuitry 30. Control circuitry in system 8 such ascontrol circuitry 16 and control circuitry 30 is used in controlling theoperation of system 8. This control circuitry may include processingcircuitry associated with microprocessors, power management units,baseband processors, digital signal processors, microcontrollers, and/orapplication-specific integrated circuits with processing circuits. Theprocessing circuitry implements desired control and communicationsfeatures in devices 12 and 24. For example, the processing circuitry maybe used in selecting coils, determining power transmission levels,processing sensor data and other data, processing user input, handlingnegotiations between devices 12 and 24, sending and receiving in-bandand out-of-band data, making measurements, and otherwise controlling theoperation of system 8.

Control circuitry in system 8 may be configured to perform operations insystem 8 using hardware (e.g., dedicated hardware or circuitry),firmware and/or software. Software code for performing operations insystem 8 is stored on non-transitory computer readable storage media(e.g., tangible computer readable storage media) in control circuitry 8.The software code may sometimes be referred to as software, data,program instructions, instructions, or code. The non-transitory computerreadable storage media may include non-volatile memory such asnon-volatile random-access memory (NVRAM), one or more hard drives(e.g., magnetic drives or solid state drives), one or more removableflash drives or other removable media, or the like. Software stored onthe non-transitory computer readable storage media may be executed onthe processing circuitry of control circuitry 16 and/or 30. Theprocessing circuitry may include application-specific integratedcircuits with processing circuitry, one or more microprocessors, acentral processing unit (CPU) or other processing circuitry.

Power transmitting device 12 may be a stand-alone power adapter (e.g., awireless charging mat or charging puck that includes power adaptercircuitry), may be a wireless charging mat or puck that is coupled to apower adapter or other equipment by a cable, may be a portable device,may be equipment that has been incorporated into furniture, a vehicle,or other system, may be a removable battery case, or may be otherwireless power transfer equipment. Illustrative configurations in whichwireless power transmitting device 12 is a wireless charging mat aresometimes described herein as an example.

Power receiving device 24 may be a portable electronic device such as awristwatch, a cellular telephone, a laptop computer, a tablet computer,an accessory such as an earbud, or other electronic equipment. Powertransmitting device 12 may be coupled to a wall outlet (e.g., analternating current power source), may have a battery for supplyingpower, and/or may have another source of power. Power transmittingdevice 12 may have an alternating-current (AC) to direct-current (DC)power converter such as AC-DC power converter 14 for converting AC powerfrom a wall outlet or other power source into DC power. DC power may beused to power control circuitry 16. During operation, a controller incontrol circuitry 16 uses power transmitting circuitry 52 to transmitwireless power to power receiving circuitry 54 of device 24. Powertransmitting circuitry 52 may have switching circuitry (e.g., invertercircuitry 61 formed from transistors) that is turned on and off based oncontrol signals provided by control circuitry 16 to create AC currentsignals through one or more wireless power transmitting coils such aswireless power transmitting coils 36. Coils 36 may be arranged in aplanar coil array (e.g., in configurations in which device 12 is awireless charging mat) or may be arranged to form a cluster of coils(e.g., in configurations in which device 12 is a wireless chargingpuck). In some arrangements, device 12 may have only a single coil. Inother arrangements, a wireless charging device such as a wirelesscharging mat may have multiple coils (e.g., two or more coils, 5-10coils, at least 10 coils, 10-30 coils, fewer than 35 coils, fewer than25 coils, or other suitable number of coils).

As the AC currents pass through one or more coils 36,alternating-current electromagnetic (e.g., magnetic) fields (wirelesspower signals 44) are produced that are received by one or morecorresponding receiver coils such as coil(s) 48 in power receivingdevice 24. Device 24 may have a single coil 48, at least two coils 48,at least three coils 48, at least four coils 48, or other suitablenumber of coils 48. In an illustrative configuration, which maysometimes be described herein as an example, device 24 has a pair ofcoils 48. When the alternating-current electromagnetic fields arereceived by coils 48, corresponding alternating-current currents areinduced in coils 48. Rectifier circuitry such as rectifier circuitry 50,which contains rectifying components such as synchronous rectificationmetal-oxide-semiconductor transistors arranged in a bridge network,converts received AC signals (received alternating-current signalsassociated with electromagnetic signals 44) from one or more coils 48into DC voltage signals for powering device 24.

The DC voltage produced by rectifier circuitry 50 (sometime referred toas rectifier output voltage Vrect) can be used in charging a batterysuch as battery 58 and can be used in powering other components indevice 24. For example, device 24 may include input-output devices 56such as a display, touch sensor, communications circuits, audiocomponents, sensors, light-emitting diode status indicators, otherlight-emitting and light detecting components, and other components andthese components (which form a load for device 24) may be powered by theDC voltages produced by rectifier circuitry 50 (and/or DC voltagesproduced by battery 58).

Device 12 and/or device 24 may communicate wirelessly using in-band orout-of-band communications. Device 12 may, for example, have wirelesstransceiver circuitry 40 that wirelessly transmits out-of-band signalsto device 24 using an antenna. Wireless transceiver circuitry 40 may beused to wirelessly receive out-of-band signals from device 24 using theantenna. Device 24 may have wireless transceiver circuitry 46 thattransmits out-of-band signals to device 12. Receiver circuitry inwireless transceiver 46 may use an antenna to receive out-of-bandsignals from device 12. In-band transmissions between devices 12 and 24may be performed using coils 36 and 48. With one illustrativeconfiguration, frequency-shift keying (FSK) is used to convey in-banddata from device 12 to device 24 and amplitude-shift keying (ASK) isused to convey in-band data from device 24 to device 12. Power may beconveyed wirelessly from device 12 to device 24 during these FSK and ASKtransmissions.

It is desirable for power transmitting device 12 and power receivingdevice 24 to be able to communicate information such as received power,states of charge, and so forth, to control wireless power transfer.However, the above-described technology need not involve thetransmission of personally identifiable information in order tofunction. Out of an abundance of caution, it is noted that to the extentthat any implementation of this charging technology involves the use ofpersonally identifiable information, implementers should follow privacypolicies and practices that are generally recognized as meeting orexceeding industry or governmental requirements for maintaining theprivacy of users. In particular, personally identifiable informationdata should be managed and handled so as to minimize risks ofunintentional or unauthorized access or use, and the nature ofauthorized use should be clearly indicated to users.

Control circuitry 16 has external object measurement circuitry 41 thatmay be used to detect external objects on the charging surface of device12 (e.g., on the top of a charging mat or, if desired, to detect objectsadjacent to the coupling surface of a charging puck). Circuitry 41 candetect foreign objects such as coils, paper clips, and other metallicobjects and can detect the presence of wireless power receiving devices24 (e.g., circuitry 41 can detect the presence of one or more coils 48).During object detection and characterization operations, external objectmeasurement circuitry 41 can be used to make measurements on coils 36 todetermine whether any devices 24 are present on device 12.

In an illustrative arrangement, measurement circuitry 41 of controlcircuitry 16 contains signal generator circuitry (e.g., oscillatorcircuitry for generating AC probe signals at one or more probefrequencies, a pulse generator that can create impulses so that impulseresponses can be measured to gather inductance information, Q-factorinformation, etc.) and signal detection circuitry (e.g., filters,analog-to-digital converters, impulse response measurement circuits,etc.). During measurement operations, switching circuitry in device 12may be adjusted by control circuitry 16 to switch each of coils 36 intouse. As each coil 36 is selectively switched into use, control circuitry16 uses the signal generator circuitry of signal measurement circuitry41 to apply a probe signal to that coil while using the signal detectioncircuitry of signal measurement circuitry 41 to measure a correspondingresponse. Measurement circuitry 43 in control circuitry 30 and/or incontrol circuitry 16 may also be used in making current and voltagemeasurements. Based on this information or other information, controlcircuitry 30 can configure rectifier circuitry 50 to help enhancewireless power reception by coils 48. For example, rectifier circuitry50 can be configured to operate in a vertical field mode in scenarios inwhich transmitted magnetic fields from device 12 are predominantlyvertical (e.g., when the coils 36 that are overlapped by coils 48 aredriven in phase) and can be configured to operate in a horizontal fieldmode in scenarios in which transmitted magnetic fields from device 12are predominantly horizontal (e.g., when the coils 36 that areoverlapped by coils 48 are driven out of phase).

FIG. 2 is a circuit diagram of illustrative wireless charging circuitryfor system 8. As shown in FIG. 2, circuitry 52 may include invertercircuitry such as one or more inverters 61 or other drive circuitry thatproduces wireless power signals that are transmitted through an outputcircuit that includes one or more coils 36 and capacitors such ascapacitor 70. In some embodiments, device 12 may include multipleindividually controlled inverters 61, each of which supplies drivesignals to a respective coil 36. In other embodiments, an inverter 61 isshared between multiple coils 36 using switching circuitry.

During operation, control signals for inverter(s) 61 are provided bycontrol circuitry 16 at control input 74. A single inverter 61 andsingle coil 36 is shown in the example of FIG. 2, but multiple inverters61 and multiple coils 36 may be used, if desired. In a multiple coilconfiguration, switching circuitry can be used to couple a singleinverter 61 to multiple coils 36 and/or each coil 36 may be coupled to arespective inverter 61. During wireless power transmission operations,transistors in one or more selected inverters 61 are driven by ACcontrol signals from control circuitry 16. This causes the outputcircuit formed from selected coil 36 and capacitor 70 to producealternating-current electromagnetic fields (signals 44) that arereceived by wireless power receiving circuitry 54 using a wireless powerreceiving circuit formed from one or more coils 48 and one or morecapacitors 72 in device 24. If desired, the relative phase betweendriven coils 36 (e.g., the phase of one of coils 36 that is being drivenrelative to another adjacent one of coils 36 that is being driven) maybe adjusted by control circuitry 16 to help enhance wireless powertransfer between device 12 and device 24. Rectifier circuitry 50 iscoupled to one or more coils 48 (e.g., a pair of coils) and convertsreceived power from AC to DC and supplies a corresponding direct currentoutput voltage Vrect across rectifier output terminals 76 for poweringload circuitry in device 24 (e.g., for charging battery 58, for poweringa display and/or other input-output devices 56, and/or for poweringother components). A single coil 48 or multiple coils 48 may be includedin device 24. In an illustrative configuration, device 24 may be acellular telephone or other portable device with a pair of coils 48.Other configurations may be used, if desired.

FIG. 3 is a cross-sectional side view of system 8 in an illustrativeconfiguration in which wireless power transmitting device 12 is awireless charging mat and in which wireless power receiving device 24 isa cellular telephone (as an example). Device 12 has housing 90 (e.g., amat housing formed form polymer, other dielectric material, and/or othermaterials). Cable 92 may be coupled to housing 90 and may provide powerto device 12. In some configurations, power may be provided by aninternal battery.

Device 24 may have a housing such as housing 96. Housing 96 and device24 may have opposing front and rear faces such as front face F and rearface R. Display 99 may be formed on front face F of housing 96 anddevice 24 and may lie in a plane that is perpendicular to the Z axis(e.g., a plane such as the X-Y plane of FIG. 3 that is parallel to theplanes including front face F and rear face R of housing 96).

The coils in devices 12 and/or 24 may have any suitable number of turnsof wire. In some configurations, the coils may be formed from turns ofwire wrapped around cores made of iron, ferrite, or other magneticmaterial.

During wireless power transmission, device 12 may use one or more coils36 to transmit wireless power signals. For example, coils 48 of device24 may overlap a pair of coils in device 12 such as coils 36′″ and 36″″.Coils 36′″ and 36″″ may be coupled to respective inverters 61. Duringoperation, control circuitry 16 may direct these respective inverters todrive corresponding coils 36′″ and 36″″ in phase (e.g., to producerespective in-phase magnetic fields B1 and B2). The magnetic fieldproduced by device 12 in this type of arrangement may predominantlyextend vertically through coils 48 parallel to the vertical Z axis ofFIG. 3. Accordingly, operation of device 12 in a configuration in whichcoils 36′″ and 36″″ are driven in phase may sometimes be referred to asoperation of device 12 in a vertical field mode. In other arrangements,control circuitry 16 may use the inverters 61 that are coupled to coils36′″ and 36″″ to drive coils 36′″ and 36″″ out of phase. The invertercircuitry of device 12 may, as an example, drive coils 36′″ and 36″″180° out of phase (e.g., to produce respective of-of-phase magneticfields B1 and B2′). This creates horizontal magnetic fields (e.g.,magnetic field lines that extends parallel to the X-Y plane of FIG. 3and parallel to the charging surface of device 12). Operation of device12 in this configuration may sometimes be referred to as a horizontalfield mode. Some wireless power receiving devices such as illustrativewireless power receiving device 24′ may have coil(s) 48 oriented toreceive horizontal magnetic fields, (e.g., horizontal field BH producedby driving coils 36′ and 36″ 180° out of phase with respect to eachother).

As these examples demonstrate, the coils 36 that are selected for use indevice 12 and the relative phase of the drive currents that are appliedto the selected coils 36 during operation affect the location andorientation of the magnetic fields produce by coils 36. The location andorientation of the magnetic fields produced by coils 36 and the locationand orientation of coils 48 relative to these fields can affect wirelesspower transmission efficiency. With an illustrative arrangement, whichis sometimes described herein as an example, device 24 has a pair ofcoils 48 coupled to adjustable rectifier circuitry 50. In thisarrangement, rectifier circuitry 50 may convert received wireless powerfrom a pair of coils (first and second coils 48) to direct-currentpower.

Rectifier circuitry 50 is adjusted dynamically by control circuitry 16to help enhance wireless power reception. As an example, controlcircuitry 16 can configure rectifier circuitry 50 for operation in avertical field mode appropriate for enhancing wireless power receptionfrom vertical magnetic fields or a horizontal field mode appropriate forenhancing wireless power reception from horizontal magnetic fields. Inthe vertical field mode, the magnetic fields B1 and B2 received by thefirst and second coils 48 are generally in phase and rectifier circuitry50 is configured to convert these in-phase wireless power signals todirect-current power. In the horizontal field mode, the magnetic fieldsB1 and B2′ received by the first and second coils are out-of-phase withrespect to each other (e.g., 180° out of phase) and rectifier circuitry50 is reconfigured to efficiently convert these wireless power signalsto direct-current power.

FIG. 4 is a circuit diagram of illustrative adjustable circuitry thatmay be used in forming adjustable rectifier 50 for wireless powerreceiving device 24. As shown in FIG. 4, wireless power may be receivedat a pair of coils 48 in device 24 such as first coil C1 and second C2.Coils C1 and C2 may, as an example, be mounted in housing 96 of device24 as shown in FIG. 3. Coils C1 and C2 may have wire turns that arewound in the same sense (e.g., both clockwise or both counterclockwise)or may, as shown in FIG. 4 have opposite winding senses (e.g., coil C1may be wound clockwise (CW) while coil C2 is wound counterclockwise(CCW)). Capacitors 72 may be interposed between coils 48 and nodes N3and N5, which serve as inputs (input terminals) for adjustable rectifiercircuitry 50. During operation of rectifier circuitry 50, direct-currentoutput voltage Vrect is produced across output terminals 76 to powerload 100 (e.g., to power input-output devices 56, to charge battery 58,and to supply power to other circuitry in power receiving device 24).Capacitor 102 may be coupled across terminals 76 in parallel with load100 to help reduce voltage ripple.

Coils C1 and C2 may be coupled in series between nodes N2 and N4. CoilC1 may have a first terminal coupled to node N2 and a second terminalcoupled to node N1. Coil C2 may have a first terminal coupled to node N4and a second terminal coupled to the second terminal of coil C1 at nodeN1. Rectifier circuitry 50 may have an array of four rectifiertransistors T1, T2, T3, and T4. Transistors T1, T2, T3, and T4 may bepassively driven field-effect transistors having body diodes coupledbetween the source-drain terminals of the transistors (e.g., transistorsT1, T2, T3, and T4 may form an array of four respective diodes). Ifdesired, transistors T1, T2, T3, and T4 may be actively driven toperform active rectification. Passively driven schemes are describedherein as an example.

Each of transistors T1, T2, T3, and T4 has a body diode having terminalscoupled to the source-drain terminals of the transistor. Transistor T1may have body diode D1 coupled in parallel with transistor switch SW1,which is open. Transistor T2 may have body diode D2 coupled in parallelwith transistor switch SW2, which is open. Transistors T3 and T4 mayrespectively have body diodes D3 and D4 coupled respectively in parallelwith transistor switches SW3 and SW4, which are open. In a passivelydriven scheme, transistors T1, T2, T3, and T4 form an array of fourrespective diodes D1, D2, D3, and D4 that are used for rectification.

Adjustable rectifier circuitry 50 has transistor T5. Transistor T5 mayinclude a body diode D5 coupled in parallel with transistor switch SW5,which may be controlled by a control signal received at the gate oftransistor T5 from control circuitry 30. The source-drain terminals oftransistor T5 may be coupled, respectively to node N1 and ground 104.Control circuitry 30 can selectively place rectifier circuitry 50 in afirst mode (sometimes referred to as the vertical field mode or verticalmode) in which transistor switch SW5 of transistor T5 is closed) and asecond mode (sometimes referred to as the horizontal field mode orhorizontal mode) in which transistor switch SW5 of transistor T5 isopen).

In the vertical mode, switch SW5 is closed and forms a short circuitbetween node N1 and ground 104 and diodes D1, D2, D3, and D4 (e.g.,transistors T1, T2, T3, and T4) of adjustable rectifier circuitry 50form two half-bridge rectifiers that are used in parallel. A firsthalf-bridge rectifier is formed from transistors T1 and T2 (diodes D1and D2) and a second half-bridge rectifier is formed from transistors T3and T4 (diodes D3 and D4). During operation in the vertical mode,current IVP flows from coils 48 through circuitry 50 during positivecycles of the received AC wireless power signal, thereby powering load100. During negative cycles of the received AC wireless power signal inthe vertical mode, current IVN flows and charges capacitors 72.

In the horizontal mode, transistor T5 has a different state (e.g.,switch SW5 is open). When switch SW5 is open, diode D5 is switched intouse between node N1 and ground 104. In this mode, transistors T1, T2,T3, and T4 (diodes D1, D2, D3, and D4) of adjustable rectifier circuitry50 form a full bridge rectifier. During positive cycles, current IHPflows through rectifier circuitry 50 and powers load 100. Duringnegative cycles, current IHN flows through rectifier circuitry 50 andpowers load 100.

Accordingly, adjustable rectifier circuitry 50 can be used to receivevertical mode magnetic fields (e.g., coil C1 may receive field B1 ofFIG. 3 and coil C2 may receive field B2 of FIG. 3) and, whenreconfigured by opening switch SW5, can be used to receive horizontalmode magnetic fields (e.g., coil C1 may receive field B1 of FIG. 3 andcoil C2 may receive field B2′ of FIG. 3). By allowing control circuitry30 to control the state of rectifier circuitry 50 (e.g., by controllingthe state of switching circuitry such as switch SW5 of transistor T5),control circuitry 30 can adjust coils 48 and rectifier circuitry 50 tohandle vertical magnetic fields or horizontal magnetic fields. Thisallows circuitry 50 to be dynamically adjusted to accommodate changes inthe magnetic field received by device 24 due to changes in the wirelesspower signal transmitted by device 12 and/or placement and orientationchanges of devices 12 and 24. The adjustability of rectifier circuitry50 therefore provides device 24 with enhanced flexibility to pick upboth horizontal and vertical magnetic flux. If desired, theincorporation of coils C1 and C2 in device 24 may allow wireless powersignals to be transmitted to accessory devices. For example, invertercircuitry in device 24 may be coupled to coils C1 and C2 and can drivethese coils to produce out-of-phase magnetic fields (e.g., horizontalmagnetic fields) that can be received by wireless earbuds or other powerreceiving devices that overlap coils 48.

Satisfactory wireless power transfer may be obtained by ensuringsatisfactory tuning of the wireless power transfer circuitry in system8. The total inductance for coils 48 coupled in series is 2L, where L isthe inductance of coil C1 and L is the inductance of coil C2. Theeffective capacitance of capacitors 72 in series is C/2, where C is thecapacitance of each capacitor 72. The resonant frequency ffb for fullbridge operation (used in horizontal mode) is thus given by equation 1.ffb=1/[2π(2L*C/2)^(1/2)]  (1)

This is the same as the resonant frequency fhb for half bridge operation(used in vertical mode) that is given by equation 2.fhb=1/[2π(L*C)^(1/2)]  (2)

Because ffb and fhb are the same, the tuning of rectifier circuitry 50does not vary even as control circuitry 30 switches rectifier circuitry50 between vertical and horizontal modes, thereby helping to ensure thatthe wireless power receiving circuitry of device 24 will not becomedetuned when switching between modes.

The foregoing is merely illustrative and various modifications can bemade to the described embodiments. The foregoing embodiments may beimplemented individually or in any combination.

What is claimed is:
 1. A wireless power receiving device comprising: ahousing having a surface; a wireless charging coil configured to receivewireless power signals; adjustable rectifier circuitry that is coupledto the wireless charging coil and that is operable in a full-bridgerectifier mode and a half-bridge rectifier mode; and control circuitryconfigured to dynamically adjust the adjustable rectifier circuitrybetween the full-bridge rectifier mode and the half-bridge rectifiermode, wherein the wireless power signals are characterized by a magneticflux direction relative to the surface and wherein the control circuitryis configured to dynamically adjust the adjustable rectifier circuitrybetween the full-bridge rectifier mode and the half-bridge rectifiermode based on changes in the magnetic flux direction relative to thesurface.
 2. The wireless power receiving device of claim 1, wherein thecontrol circuitry is configured to dynamically adjust the adjustablerectifier circuitry between the full-bridge rectifier mode and thehalf-bridge rectifier mode based on changes in the wireless powersignals received by the wireless charging coil.
 3. The wireless powerreceiving device of claim 2, wherein the adjustable rectifier circuitryis configured to output a rectified voltage based on the wireless powersignals received by the wireless charging coil.
 4. The wireless powerreceiving device of claim 1, wherein the wireless power signals aregenerated from a location relative to the surface and wherein thecontrol circuitry is configured to dynamically adjust the adjustablerectifier circuitry between the full-bridge rectifier mode and thehalf-bridge rectifier mode based on changes in the location.
 5. Thewireless power receiving device of claim 1, wherein the wireless powersignals are generated by a wireless power transmitting device having acharging surface and wherein the control circuitry is configured todynamically adjust the adjustable rectifier circuitry between thefull-bridge rectifier mode and the half-bridge rectifier mode based onchanges in placement of the wireless power receiving device on thecharging surface.
 6. The wireless power receiving device of claim 1,further comprising: a switch having a first terminal coupled to thewireless charging coil and having a second terminal coupled to ground,wherein the switch is controlled by the control circuitry.
 7. Thewireless power receiving device of claim 6, wherein the controlcircuitry is configured to deactivate the switch in the full-bridgerectifier mode and to activate the switch in the half-bridge rectifiermode.
 8. An accessory comprising: a housing having a surface; a coilconfigured to receive wireless power signals; adjustable rectifiercircuitry that is coupled to the coil and that is operable in afull-bridge rectifier mode and a half-bridge rectifier mode; and controlcircuitry configured to dynamically adjust the adjustable rectifiercircuitry between the full-bridge rectifier mode and the half-bridgerectifier mode based on changes in the wireless power signals receivedby the coil, wherein the wireless power signals are generated from alocation relative to the surface and wherein the control circuitry isconfigured to dynamically adjust the adjustable rectifier circuitrybetween the full-bridge rectifier mode and the half-bridge rectifiermode based on changes in the location.
 9. The accessory of claim 8,wherein the adjustable rectifier circuitry is configured to output arectified voltage based on the wireless power signals received by thecoil.
 10. The accessory of claim 8, wherein the control circuitry isconfigured to communicate with a portable electronic device to pair theaccessory with the portable electronic device.
 11. The accessory ofclaim 8, wherein the control circuitry is configured to communicate witha portable electronic device having a display.
 12. The accessory ofclaim 8, further comprising: a switch having a first terminal coupled tothe coil and having a second terminal coupled to ground, wherein theswitch is controlled by the control circuitry.
 13. The accessory ofclaim 12, wherein the control circuitry is configured to open the switchin the full-bridge rectifier mode and to close the switch in thehalf-bridge rectifier mode.
 14. An apparatus comprising: a wirelesscharging coil; reconfigurable rectifier circuitry that has fourswitches, that is coupled to the wireless charging coil, and that isoperable in a first rectifier mode and a second rectifier mode; anadditional switch coupled to the wireless charging coil; and controlcircuitry configured to dynamically adjust the reconfigurable rectifiercircuitry between the first and second rectifier modes by controllingthe additional switch.
 15. The apparatus of claim 14, wherein thereconfigurable rectifier circuitry forms a full-bridge rectifier in thefirst rectifier mode and forms at least one half-bridge rectifier in thesecond rectifier mode.
 16. The apparatus of claim 14, wherein thereconfigurable rectifier circuitry forms a full-bridge rectifier in thefirst rectifier mode and forms a non-full-bridge rectifier in the secondrectifier mode.
 17. The apparatus of claim 14, wherein the controlcircuitry is configured to turn off the additional switch in the firstrectifier mode and to turn on the additional switch in the secondrectifier mode.
 18. The apparatus of claim 14, wherein the additionalswitch has a first terminal coupled to the wireless charging coil andhas a second terminal coupled to ground.
 19. The apparatus of claim 14,wherein the control circuitry is configured to dynamically adjust thereconfigurable rectifier circuitry between the first and secondrectifier modes based on changes in wireless power received by thewireless charging coil.